François Leroy , Marta Jarzyna, Petr Keil

Acceleration of human activities over the past century might have caused a corresponding acceleration in the decline of species’ abundances, but this has not been empirically assessed. Further, the temporal dynamics of abundance arises from a complex interaction between recruitment and loss of individuals, which remains unexplored across large spatial scales. We address these gaps by examining temporal changes, acceleration, deceleration, and vital processes (i.e. recruitment and loss) of abundance across much of the North American avifauna from 1987 to 2021. We confirm the continent-wide decline of bird abundance, and pinpoint the regional hotspots of acceleration of this decline in the Mid-Atlantic region, Midwest, and California, matching broad spatial patterns of human activities. We further reveal that the increasing loss rate of individuals is the primary process responsible for the acceleration of abundance decline in California and the Midwest, whereas a decrease in recruitment rate is also observed in the Mid-Atlantic. Finally, our models show that 67% of bird species and 95% of families with increasing abundances are concurrently experiencing a decline in recruitment rate. This underscores the need for conservation policies even for species that appear to be thriving. Moreover, simply preventing loss may not suffice, as we also need policies that enhance recruitment.